Permanent magnet for beam tubes



Dec. 17, 1957 J. s. HlcKEY, JR

PERMANENT MAGNET FOR BEAM TUBES y Filed oct. 15. 1954 IININIPIIIJHHH.

l o -WUI a INVENTOR. Jah/7 S Hic/(ey, Jr.

United States Patent PERMANENT MAGNET FOR BEAM TUBES John S. Hickey, Jr., Ballston Lake, N. Y., assigner, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application October 15, 1954, Serial No. 462,637

3 Claims. (Cl. S15-3.5)

This invention relates to electron beam tubes and, more particularly to the magnetic field used in these tubes for focusing or collimating the electron beam.

Generally, two sources of magnetic fields have been used in electron beam tubes for focusing and collimating the electron stream, these sources being the well-known and permanent magnets which commonly are referred t as electron lenses. In the electro-magnetic lenses, solenoids have been used successfully for providing the magnetic field, although there are certain disadvantages arising in conjunction with large coil design due to Weight and construction cost considerations that sometimes become prohibitive. Furthermore, increasing the strength of the field introduces the need for high temperature insulation as well .as the need for heat transfer apparatus, such as air blowers or liquid coolants t-o dissipate the heat. Permanent magnets have been used for the purpose of eliminating the D. C. required in electro-magnets but they too have met with limited success, principally because of end flux leak-age and the failure to provide a uniform magnetic field throughout the magnet.

Accordingly, a principal object of this invention is to provide a permanent magnet capable of producing a uniform magnetic field, and a corollary object is to provide such a magnet having an increased saturation value.

A further object is to provide a permanent magnet in which the entire magnet can be utilized to produce the magnetic eld and thereby conserve the use of normally expensive magnet material and reduce the tube size.

Still another object is to provide a permanent magnet which will increase the over-all efciency of travelingwave tubes.

These and other objects will become apparent from the detailed description and the accompanying drawing.

The objects of this invention are generally accomplished by the discovery that a permanent magnet having a general ellipsoidal configuration can be more uniformly magnetized along the axis of revolution. Such a configuration reduces field distortion and `demagnetization of the magnet due to end flux leakage which, in a uniform cylindrical magnet, causes the central portion to saturate before the ends and results in a lower flux field in the center. This effect in the uniform magnet reduces the total saturation level of the magnet.

In a specific embodiment, the permanent magnet 1s preferably constructed of Alnico material and is prov1ded with a bore along its axis of revolution to house a suitable electron gun for immersion in the magnetic field. Another important feature of the invention resides in the employment of pole pieces for the magnet ends. It has been found that due to end flux leakage, axial field intensity of the ellipsoidal magnet is greatly reduced at its ends and, as a consequence, utilization of only that part of the magnet having a uniform intensity would result in the waste of a considerable portion of the magnet. Mounting the pole pieces near the ends of the magnet enables the effective use of the entire magnet and the conservation of the non-productive ends of the magnet ends which formerly were present but of no value. In a manner similar to the magnet, the pole pieces preferably are tapered toward their free ends to reduce end leakage.

In the drawings, Fig. l is a partial longitudinal section of a traveling wave tube formed in accordance with the preferred embodiment `of the invention; and Fig. 2 is a sectional view taken along line II-II of Fig. l.

Referring to the drawing, reference numeral 10 generally indicates a traveling-wave tube -that is particularly suitable for radio-frequency amplification at very high frequencies, such as 3000 megacycles and higher. Tube 10 -comprises a conventional electron discharge device gun 12 positioned in the bore of a permanent magnet 14 for immersion in its magnetic field and, although the magnet construction forms the subject matter of this invention, a preliminary explanation of the tube operation should afford a clearer understanding of the principles involved.

Gun 12 includes an evacuated elongate envelope 15 containing a heater 16, a cathode 18, an anode 20, and a collector 22, the latter being located at the opposite end Iof the tube and usually at the same potential as -the anode. The electron gun produces a cathode-ray beam, indicated by broken line 2S, which is shot through a closely wound helix 26 that may be positioned inside or outside envelope 15. Permanent magnet 14 produces an axial magnetic field around the electron ilow that functions to collimate -or focus the electron beam and maintain the beam diameter small for guidance through the center of helix 26.

The signal to be amplied yby the electron beam is introduced through a coaxial line or wave guide at one end of the helix adjacent the gun and the amplified output is withdrawn through a wave guide at the -other end of the gun, the details of the wave guide input and output connections not being shown. A-s is well known, the input signal produces a wave that propagates along the helix with the production of a traveling axial electrostatic field inside the helix. When the velocity of the electrons travcling through the helix is slightly greater than the axial velocity of the wave on the helix, an interaction takes place between the moving axial electrostatic field `of the helix wave rand the moving electrons such that the wave is amplified. The characteristics of a traveling wave are well known and need not be related in this application.

The effectiveness of the focusing action by magnet 14 is dependent on the strength and the uniformity of its magnetic field. It is known that leakage ux from the material near the circumference will vary the flux density and the field. To obtain a maximum saturation level of the magnetic material during magnetization, magnet 14 is constructed with a configuration generally of an ellipsoid. Such a magnet construction provides uniform demagnetization due to side leakage and prevents the saturation of the center section before the ends which would otherwise result in a lower field in the center and a lower Isaturation level of the magnet. Thus, uniform saturation of the magnet throughout its length enables a higher saturation level to be reached.

An additional flux leakage effect to be considered occurs at both ends of the magnet. It has been found that a substantial decrease in the field intensity occurs at the ends of the magnet and, in fact, results in field reversal. In practice, this would necessitate the construction of a magnet longer than is actually necessary in order to use the intermediate portion of the magnet that will produce a uniform held intensity. This construction represents an uneconomical use of expensive permanent magnet material and probably also will increase the size of the tube.

Therefore, another important feature of the invention is the provision of hollow steel pole pieces 28 and 30, one

mounted at each end of magnet 14. It has been discovered that these pole pieces can replace those end portions of the magnet that do not produce a usable field and thereby conserve up to 25 percent of the amount of magnet as Well as prevent a field reversal. The pole pieces are tapered to reduce end leakage. Pole piece 28 is provided with a cap end having an axial opening 32 for the passage of the various electrical leads to the gun, and pole piece 30 may be formed integral with collector 22.. The envelope is shown supported in the pole pieces by steel ferrules 32 and 34.

Magnet 14 is preferably made of Alnico, being one type of permanent magnet in the aluminum-nickel-iron series of alloys. lt is probable that a usable axial field of 500 gauss (H) can be obtained from an ellipsoid magnet constructed according to the invention of Alnico V, and a suggested magnet configuration has the following longitudinal dimension: section A=28 inches; section 13:12 inches; section C=4 inches, and section D=4 inches. The diameters of the magnet sections are as follows: bore diameter 2% inches, diameter of section B 6 inches, tapering down to a diameter of 5 inches at the junction of section C and D, and an end diameter of 31/2 inches. lt should be noted that in an axially magnetized magnet as illustrated the bore diameter is small as compared to the magnet dimensions, and may be considered as a Gaussian slit. To obtain a field of 750 H or more, it is believed that the only feasible magnet material is Alnico VII.

Due to manufacturing limitations, the magnet may be made in four symmetrical quarters, for example, divided in half both longitudinally and transversely, and suitably encased in a sleeve prior to magnetizing (not shown).

According to the invention, an axially magnetized magnet is provided having an ellipsodial configuration that will produce a maximum and uniform magnetic field. Full utilization of the entire length of the magnet is accomplished by pole pieces that, in addition, contribute to the uniform field. It is apparent, therefore, that improved results can be attained with a smaller and less expensive magnet.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that Within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

I claim:

1. A traveling-wave amplifier tube comprising an electron beam gun, a permanent magnet having an ellipsoidal Iconfiguration and magnetized along its axis of revolution for providing a magnetic field for focusing the beam, `said magnet having a metal pole piece mounted at each end, said magnet having a longitudinal bore to house and immerse the electron beam gun in the axial field.

2. A traveling-wave amplifier tube comprising an electron beam gun, a permanent magnet having an ellipsoidal configuration and magnetized along its axis of revolution for providing a magnetic field for focusing the beam, said magnet having a metal pole piece mounted at each end, said magnet having a longitudinal bore to house and immerse the electron beam gun in the axial field, and said pole pieces each being tapered for forming a continuation of said ellipsoidal configuration of the magnet.

3. A traveling-wave amplifier tube comprising an electron beam gun, an elongate permanent magnet having an ellipsoidal configuration and a metal pole piece mounted at each end of the elongate magnet, said magnet having a longitudinal bore and also having a radial depth of permanent magnet material which is greater at the midpoint of the elongate magnet than adjacent the ends of the magnet, said pole pieces each being tapered for forming a continuation of said ellipsoidal configuration, and said electron beam gun `including ray beam-producing means mounted in one of said pole pieces, and collector means mounted in the other pole piece, said permanent magnet being magnetized along its axis of revolution for providing a magnetic field for focusing the beam.

References Cited in the file of this patent UNITED STATES PATENTS 2,159,534 Ruska May 23, 1939 2,259,531 Miller et al. Oct. 2l, 1941 2,300,052 Lindenblad Oct. 27, 1942 2,608,668 Hines Aug. 26, 1952 FOREIGN PATENTS 901,325 Germany Jan. 11, 1954 

